The recovery of methane from carbon dioxide-rich natural gas reserves is a technical and economic challenge. Some reservoirs around the world may contain up to about 90% carbon dioxide. Generally, carbon dioxide must be separated from the natural gas stream because carbon dioxide contains no thermal heating value and has acidic properties which can corrode natural gas equipment.
There are numerous methods available for removing carbon dioxide from natural gas streams. Most commonly used are processes that operate through removal of carbon dioxide by chemical absorption/desorption, or cryogenic fractionation, both of which are capital and energy intensive for dilute streams. Natural gas is treated in very high volumes making even slight differences in the capital or operating cost of treatment systems significant factors in selecting a process technology.
The chemical absorption process utilizes a chemical solvent such as ethanolamine to chemically bind the carbon dioxide for removal. This generally occurs by passing the unpurified natural gas stream through a packed or trayed tower in which the chemical solvent is flowing countercurrently to the natural gas stream. The chemical solvent will bind and remove the carbon dioxide and a purified natural gas stream emerges from the top of the tower.
The chemical solvent, which is then rich in carbon dioxide, must be regenerated so that the solvent can be reused or recycled. This typically occurs by lowering the pressure and raising the temperature of the carbon dioxide rich solvent stream in a regeneration tower. This tower generally consists of a reboiler and a reflux condensing system. Heat is added in the reboiler to break the carbon dioxide-solvent bonds and the reflux system condenses any solvent vapors that might escape with the carbon dioxide gas. The purified carbon dioxide emerges from the top of the tower at a pressure substantially lower than the original natural gas stream.
Cryogenic processing is a physical process in which subambient temperatures are employed to bring about a separation between different components in a gas mixture. Cryogenic plants are not used very widely because they are expensive and complicated. A particular complication is the need for significant pretreatment to remove water vapor and heavy hydrocarbons and aromatics to avoid the freezing of these components in the cryogenic section of the plant, which typically operates at temperatures down to -150.degree. C. The degree of pretreatment is usually far more elaborate and the demands placed upon it far more stringent than would be required to render the gas acceptable in a pipeline grid.
The methods described above and all those known to Applicants generally involve the removal of carbon dioxide from natural gas streams that are fairly rich in methane. For dilute gas streams, i.e. those comprising less than about 50% methane, these processes generally are inefficient. Accordingly, there remains a need for a more efficient and cost effective means of methane recovery from fields of carbon dioxide-rich natural gas reserves, which historically have presented an economic and technical challenge.